A method of conjugating a polymer material and an inorganic material such as a metal oxide with each other to impart characteristics of the inorganic material to the polymer material is significantly useful for preparing a new functional material. In general, in a case where such a conjugation is attempted, for example, various methods such as modifying a surface of an inorganic material such as a metal oxide to improve compatibility between the inorganic material and a polymer material or obtaining a composite body through polymerization of raw materials conjugated in advance are used.
Among polymer materials, a silicone resin has not only excellent weather resistance such as heat resistance or cold resistance but also excellent electrical properties, low toxicity, and the like. Therefore, a silicone resin is used as materials of various fields including cosmetic materials, medical materials, and electric and electronic materials. In addition, recently, the transparency of a silicone resin has also attracted attention, and thus a silicone resin has also been used in a member requiring transparency such as a transparent sealing member of a light emitting diode. Examples of characteristics required for this use include optical characteristics, such as transparency and refractive index, and mechanical characteristics such as hardness.
For example, as techniques of conjugating a silicone resin and an inorganic material such as a metal oxide with each other to improve optical characteristics and mechanical characteristics, many techniques, for example, a transparent coating material for a photocatalyst (Patent Literature No. 1) which is obtained by conjugating titanium oxide particles with a silicone resin binder, a cosmetic material (Patent Literature No. 2) which is obtained by treating a surface of titanium oxide powder with an oil solution, a silicone compound, a fluorine compound, or the like in order to impart transparency, and the like are disclosed.
However, when a silicone resin and an inorganic material such as a metal oxide are conjugated with each other, in a case where any treatment is not performed on a surface of the metal oxide which is originally hydrophilic, the hydrophilic metal oxide does not have sufficient affinity to the highly hydrophobic silicone resin, and metal oxide particles agglomerate. As a result, there is a problem in that a uniform composite body cannot be obtained. Therefore, a conjugation method is discussed in which a surface of a metal oxide is modified with a hydrophobic dispersant, a reactive silicone, or the like to be hydrophobized such that affinity between the metal oxide and a silicone resin material is improved.
In such a composite body of a silicone resin and surface-modified metal oxide particles, in a case where a content rate of the metal oxide particles is increased to improve physical characteristics such as optical characteristics affected by the content of the metal oxide, there is a problem in that a content rate of a surface modifier is also increased along with an increase in the content rate of the metal oxide particle. In particular, in a case where metal oxide particles having a small particle diameter are used in order to maintain the transparency of a composite body, this problem becomes significant. Accordingly, a content rate of a silicone resin in a composite body may become excessively low, and thus hardness, flexibility, and the like which are largely affected by the silicone resin may be insufficient.
Further, it is more difficult to uniformly and finely disperse metal oxide particles of titanium oxide, zinc oxide, zirconium oxide, or the like having a relatively high refractive index and a high specific gravity in a silicone resin for conjugation, as compared to silica particles having the same siloxane skeleton as silicone and having a relatively similar refractive index to silicone. Accordingly, in a member having a relatively long optical path length such as a transparent sealing member, there is a problem in that it is difficult to simultaneously realize transparency and a high refractive index.
In order to solve the above-described problem, a technique (Patent Literature No. 3) relating to a metal oxide particle-silicone resin composite body having transparency, optical characteristics such as refractive index, and mechanical characteristics such as hardness in which surface-modified metal oxide particles are dispersed in a silicone resin is disclosed, the surface-modified metal oxide particles being obtained by modifying surfaces of metal oxide particles with a silane compound having a silicon-hydrogen bond (hydrosilyl group) and a saturated alkylsilane having a hydrophobic functional group.